1. Field of the Invention
The present invention relates to an optical element and optical apparatus and, more particularly, to an optical element and optical apparatus suitably applicable, for example, to video cameras, still video cameras, copiers, and so on.
2. Related Background Art
An inverted, real image system without intermediate image formation is principally used as an optical system in the conventional optical apparatus for forming an image of an object on the final image plane.
On the other hand, an optical system with intermediate image formation is used when a small cross section is desired for the optical system or when an erect image is demanded.
FIG. 6A to FIG. 6C are conceptual drawings of such optical systems. FIG. 6A and FIG. 6B are conceptual drawings where intermediate image formation is brought about in a coaxial system.
In FIG. 6A, reference numeral 1 designates the object plane and 5 an optical system, the system 5 being comprised of lens systems L1, L3, and L2. Numeral 2 denotes an intermediate image plane, the plane 2 being located inside the lens system L3. Numeral 3 represents the final image plane.
In the same figure, lights from the object 1 are condensed by the lens system L1 to be focused on the intermediate image plane 2 inside the lens system L3 and form an inverted object image (intermediate image) on the plane 2, and thereafter the lights are condensed by the lens system L2 to be focused on the final image plane 3 and form an erect object image thereon.
In this example, the lens system L1 is composed of an object-side imaging element for forming the image of the object 1 on the intermediate image plane 2 in the lens system L3, and the lens system L2 is composed of an image-side imaging element for reimaging the image on the intermediate image plane 2, on the final image plane 3.
Each of these imaging elements composes a part of the imaging optical system 5. The lens system L3 herein conceptually represents the optical system corresponding to a field lens, a prism block, or the like.
FIG. 6B is a major-part sectional view of an erect, real image forming system using a medium having a nonuniform index profile. In the same figure lights from the object 1 are condensed by a front portion 10 of the imaging optical system (optical system) 5 to form an image on the intermediate image plane and thereafter they are focused by a rear portion 11 of the imaging optical system 5 to form an object image on the final image plane 3. In this case, the front portion 10 composes the object-side imaging part element and the rear portion 11 the image-side imaging element. These together compose the imaging optical system 5. A lens array, in which lenses composed of such erect 1:1 imaging systems using the medium with the nonuniform index profile are arrayed, is used in the copiers and other equipment.
FIG. 6C is a major-part schematic diagram to show an optical apparatus having a non-coaxial, optical system and an intermediate image plane therein, as disclosed in Japanese Patent Application Laid-open No. 8-292371. In the same figure, lights from the object 1 pass through an aperture stop 4 and thereafter are incident to an entrance surface 10-1 of optical element 5 to be refracted into the element 5. They are then reflected by a concave, reflective surface 10-2 and thereafter focused on the intermediate image plane 2. The lights from the intermediate image plane 2 are thereafter reflected by reflective surfaces 11-1, 11-2, 11-3 to propagate inside the element 5 and emerge from an exit surface 11-4, then forming the object image on the final image plane 3.
In FIG. 6C, the concept of reference axis (8-1 to 8-5) is employed in correspondence to the optic axis of the coaxial system. This reference axis is defined as an optical path of a reference-wavelength ray traveling via the center of the object 1 and the center (of the aperture) of the stop 4.
This optical system is called an off-axial, optical system because the optical system includes a surface in which at an intersecting point between the reference axis corresponding to the optic axis and the component surface the reference axis does not agree with the normal to the surface but makes a finite angle other than 0 therewith (the definition of the off-axial, optical system). Surfaces of this type are called off-axial surfaces or off-axial curved surfaces. In this case, the imaging optical system 5 is also composed of a front element 10 (the surfaces 10-1, 10-2 ) composing the object-side imaging element and a rear element 11 (the surfaces 11-1, 11-2, 11-3, 11-4 ) composing the image-side imaging element, the elements 10, 11 being incorporated.
The non-coaxial, off-axial, optical system is described in detail in Japanese Patent Application Laid-open No. 9-5650, including the setting method of surface shapes and the calculating method of paraxial amounts together with properties of the off-axial, optical system.
Although FIGS. 6A, 6B, 6C show the examples where single intermediate imaging is achieved, for simplicity, there are known systems involving a plurality of (two or more) intermediate imaging procedures.
The optical systems involving the intermediate imaging often adopt a designing approach for retaining the imaging performance, including spherical aberration, on the intermediate image plane and maintaining this performance so as to keep the imaging performance on the final image plane. This approach is conceptually easy to understand and also easy to design, because the approach includes use of some normal designing techniques. In the case of design using the automatic designing approach, the design is sometimes conducted normally with no consideration on imaging characteristics on the intermediate image plane at all but with only consideration on the imaging characteristics on the final image plane.
Even in such cases, however, one can often reach a solution that retains some imaging characteristics, including spherical aberration, on the intermediate image plane.
In general, there exist foreign materials such as dust particles, and bubbles in the optical elements. Sizes of these are various, but the standards of Glass Industry Association include the standard of bubble for 30 μm and greater bubbles in the optical elements. In the case of special, visual type lenses, even a bubble or a particle as fine as 30 μm or less would pose a problem, so that special inspection is needed for such lenses.
The limit of detection of a bubble or a particle by naked eyes is about 5 μm, and those having the size of about 100 μm first become capable of visual detection. For the surface reflectors, a flaw, a deposit, or the like on the surface is also a cause of degrading the optical performance and inspection thereof is necessary. The sizes of the bubbles and particles in the optical elements and the widths of flaws or the sizes of the deposits or the like on the reflective surfaces, which could pose a problem, vary depending upon design specifications and manufacturing cost, but, with emphasis on the manufacturing cost, a product is determined to be defective if there are many 100 μm- and greater bubbles or particles, or flaws, deposits or the like on the reflective surface, which can be detected visually. With emphasis on the optical performance, because the bubbles, particles, deposits, and flaws having the sizes or widths of 10 μm or less are considered not to affect the optical performance, it seems valid to make the determination of defective if there are many bubbles, particles, deposits, flaws, etc. having the sizes of 10 μm and above. As described above, the sizes of the bubbles, particles, deposits, etc. to be criteria of inspection are approximately 10 to 100 μm.
The sizes of the bubbles, particles, deposits, etc. posing the problem will be described from the aspect of specifications of product. In general, the sizes of the bubbles, particles, deposits, etc. posing the problem on an image pickup device vary depending upon types of products, types of images, or individuals, but an eyesore often starts when the size of an image of the bubble, particle, deposit, etc. on a photoreceptive surface of the image pickup device in an in-focus state exceeds approximately five times (5b) the length of the minimum resolution (b) given by the size of pixels of the image pickup device or the like. This numeral of 5 is one figured out by experiments with plural types of images against plural subjects and corresponds to the fact that a drop of one pixel or so is not offensive to the eye, but about five times one pixel often becomes offensive.
Accordingly, in FIGS. 6A to 6C, when the intermediate image plane 2 is imaged on the final image plane 3 where the image pickup device is located and when β11 represents the image magnification of the lens system 11 of the image-side imaging element, the size of a noise source posing the problem near the intermediate image plane 2 is not less thean approximately the following:5b/|↑11|  (Eq 1).
In this equation, |β11| indicates an absolute value of the image magnification β11 of the lens system 11 being the image-side imaging element. For example, supposing the pixel size of CCD being the image pickup device is 5 μm square and β11 is 1, the size of the noise source posing the problem near the intermediate image plane is not less than 25 μm.
Particularly, in the optical systems arranged to form the intermediate image plane in the lens system, in the optical block, or the like, if there exists a noise source, such as a dust particle, a bubble, or a flaw, irrelevant to an image (object image) desired to be transmitted to near the intermediate image position, the noise source will be a cause of heavily degrading the optical performance.
If the noise from such a noise source overlies an image (signal) on the photoreceptive surface of the image pickup device being the final image plane, there will arise a problem that the image becomes harder to see. Specific examples of the dust, bubble, or flaw irrelevant to the desired-to-transmit image include the dust particles or bubbles (indicated by NO in the drawing) in the internally solid optical member near the intermediate image plane 2, as shown in the system (coaxial system) of FIG. 7A and in the system (non-coaxial system) of FIG. 7B, and the flaws (indicated by NO in the drawing) on a component surface of the optical system near the intermediate image plane 2, as shown in the system (coaxial system) of FIG. 7C and in the system (non-coaxial, off-axial, optical system) of FIG. 7D.
Another special noise source is a streak pattern C of steps of a Fresnel lens or diffraction type lens where the Fresnel lens or diffraction type lens is used as a field lens 12 as shown in FIG. 7E.
In general, a popular method for making such a noise source as the dust, bubble, or flaw inoffensive was a method for designing the intermediate image plane to be located in air and thereby keeping the noise source, existing in the optical medium, on the surface thereof, or on the reflective surface, in a defocus state, as described, for example, in Japanese Patent Application Laid-open No. 6-265814.
However, the problem was that this method for making the noise source inoffensive by defocus was not applicable to optical systems downsized by integrally forming the optical medium and forming the intermediate image therein as described in Japanese Patent Application Laid-open No. 8-292371, because there was no air layer inside.
Japanese Patent Application Laid-open No. 6-265814 also describes an example in which aberration such as spherical aberration is intentionally brought about on the intermediate image plane, so as to make the streaks of the Fresnel lens as a field lens inoffensive. This method for generating aberration, however, generates the aberration by use of a rotationally symmetric system, so that the aberration is the third or higher order aberration. This method thus had the problem that the effect appeared weaker in a dark optical system, particularly in an optical system stopped down or the like, than in the method by defocus which worked in the first order.
In the case where the intermediate image moves depending upon zoom positions as described in Japanese Patent Application Laid-open No. 8-292372, even if the intermediate image at a certain zoom position is in air, the intermediate image plane at another zoom position could be near the surface of the component surface or in the optical medium. In that case there arose the problem that the noise source was unable to be made inoffensive at the zoom position.
In the case where the focal length is constant but the object distance varies, the final image plane can be kept aligned by focusing, but the position of the intermediate image plane also varies on that occasion. Therefore, even if at a certain object distance the intermediate image is in air by defocus, the intermediate image plane at another object distance could be near the surface of the component surface or in the medium. In that case there arose the problem that the noise source was unable to be made inoffensive at that object distance.